In the case of radiation therapy in which treatment is implemented by use of radiations, it is required that a dose with which a sufficient therapy effect is demonstrated is provided to a focus such as a cancer while normal tissues are suppressed as much as possible from being exposed to the radiations. Accordingly, in general, a treatment plan is required in which before a therapy is implemented, it is determined and decided from which direction and at which intensity level radiations should be irradiated, by use of image data taken by a diagnostic imaging apparatus such as an X-ray CT apparatus and based on the result of a dose distribution simulation or the like.
In general, such a treatment plan is implemented by use of software that operates on a computer system. In a treatment plan, at first, there is set a three-dimensional region for a focus and its peripheral normal tissues on which attention is to be placed, by utilizing image data, and then the coordinates thereof are stored in a memory. Next, based on an irradiation coverage (referred to as an irradiation field) determined in accordance with the size of the focus and a tentatively decided irradiation direction and irradiation intensity, a three-dimensional dose distribution in a human body is calculated pursuant to a given physical model and by use of the image data.
The result obtained in such a manner is evaluated by use of various kinds of evaluation methods. The evaluation methods include, for example, a DVH (Dose Volume Histogram) which is a graph representing a relationship between a dose and the volume of a tissue having the value of the dose, an isodose chart in which a dose distribution is superimposed on a human body tomogram, a three-dimensional display in which a dose distribution, which is kept as three-dimensional data, is superimposed on a human body tissue and the superimposed chart is expressed in a translucent and a three-dimensional manner, and the like. In the case where it is determined through these methods that the dose distribution is a desirable one, the tentatively decided irradiation direction and irradiation intensity are adopted for the therapy; otherwise, the irradiation direction and the irradiation intensity are again decided and the dose distribution is calculated once again; then, the result is evaluated. In general, in a treatment plan, such work is repeated so that the irradiation direction and the irradiation intensity to be adopted for a therapy are decided.
Patent Document 1 discloses a three-dimensional image processing method through which in order to visually evaluate a treatment plan, there is outputted a three-dimensional display where a human body tissue and a dose distribution, which is obtained by a calculation, are superimposed on each other. A three-dimensional display in which a dose distribution obtained through a calculation is superimposed on a human body tissue has been rendered with the dose distribution as a region of 10% or larger target dose or with the dose distribution as a region of 95% or larger target dose, when the maximum dose value is 100% target dose; in other words, the three-dimensional display has been rendered with different dose-distribution rendering conditions. As a result, for example, it has been observed that a focus is included in the region of 10% or larger target dose and is slightly out of the region of 95% or larger target dose, so that the dose for the focus has been evaluated.